1. Field of the Invention
The present invention relates to a printing apparatus, particularly to a printing apparatus which prints using a plurality of printheads.
2. Description of the Related Art
A printer which prints information such as a desired character or image on a sheet-like printing medium such as paper or a film is widely used as an information output apparatus for a word processor, personal computer, facsimile apparatus, or the like.
Various methods are known as printing methods of a printer. Recently, an inkjet method especially receives attention because it allows non-contact printing on a printing medium, can easily print in color, and is very quiet. The arrangement according to the inkjet method popularly adopts a serial method because of low cost, easy downsizing of the apparatus, and the like. In this method, a printhead for discharging ink in accordance with a printing instruction is mounted, and prints while reciprocally scanning the printhead in a direction perpendicular to the printing medium conveyance direction.
The inkjet printing apparatus (to be referred to as a printing apparatus) implements higher-resolution image printing by decreasing the ink discharge amount per dot while increasing the integration degree of nozzles for discharging an ink droplet. To obtain higher image quality, a variety of techniques have been proposed, including a technique of simultaneously printing with inks of four basic colors (cyan, magenta, yellow, and black), light color inks prepared by decreasing the dye concentrations of these color inks, and special color inks of red, green, blue, and the like.
Even a decrease in printing speed which may occur along with improvement of the image quality has been suppressed satisfactorily by increasing the number of integrated printing elements, increasing their drive frequency, and introducing a printing technique of printing in reciprocal scanning of the printhead, such as reciprocal printing. In a printhead including many printing elements, a printing element failure might occur over time in accordance with the use frequency. Even though many printing elements are normal, if only one printing element fails, the image quality degrades.
Especially in photographic image printing requested recently, occurrence of only some printing element failures makes actual use of the printhead difficult. As a countermeasure against this, various printing element failure detection methods, recovery methods or printing methods corresponding to detection results, and the like have already been proposed (see Japanese Patent Laid-Open Nos. 11-188853 and 2001-315363).
As a conventional element failure detection method, an ink discharge state from a nozzle of a printhead is detected using a photosensor. There is also a method of detecting an ink discharge state from a nozzle of a printhead using an electrode which detects, via ink, a voltage change between a printing element which receives energy to discharge ink and a driving element for driving the printing element. There are also known a method of printing a stepwise pattern on a printing medium using all printing elements and determining an unprinted portion of the pattern using a photosensor, and a method of discharging a charged ink droplet onto an electrode and detecting an ink discharge state from a potential change of the electrode at that time.
However, the conventional detection methods cannot determine whether the detection result is derived from a printing element itself or merely clogging of a nozzle with an ink droplet, dust, or the like.
There are also proposed a method of identifying a printing element failure in inspection before shipping of a printhead and storing the information into the printhead, and a method of storing in advance information about an element which is highly likely to fail, and decreasing the drive frequency of this element. In this case, no problem occurs upon shipping of a printhead, but neither the use environment nor aging of the printhead is satisfactorily coped with.